Overview of the BaBar Simulation

Introduction

Simulation Architecture

The simplified architecture of the BaBar simulation is shown below.





There are four main stages in producing a simulated event:

1. Generation of the physics event
   • The physics of the e+ e- collision is performed by one of several event generators. The generators provide a set of four-vectors which represents the final state of the collision near the e+ e- interaction point. B0B0bar events, for example, are simulated by the routine EvtGen which is part of the GenFwkInt framework package. Other events which are routinely generated are dimuons (Bkqed) and Bhabha scattering (Bhwide). More information about event generators can be found at Event Generators .
   
   
2. Particle transport and hit scoring
   • The four-vectors from the generator stage are transported through the simulated detector, where energy loss, production of secondaries, multiple scattering and decay can occur. As these particles pass through sensitive regions of the detector, energy, charge and angle information is used to calculate positions and idealized energy deposits in the detector. These quantities, referred to as "GHits" are stored in persistent containers in the Objectivity database for later use in calculating the detector response. The software package which handles the above tasks is called Bogus (BaBar Object-oriented Geant4-based Unified Simulation). Bogus is an application layer built on the Geant4 toolkit. Geant4 provides the physics processes listed above as well as the means to build the detector geometry. It also provides several particle transport methods, but currently BaBar uses its own customized transportation. Source code, documentation and user support for Geant4 can be found at Geant4 . An overview of Bogus can be found at Bogus.
   
   
3. Detector Response and Background Mixing
   • At this stage idealized GHits are retrieved from the database and digitized, that is, transformed into realistic signals which mimic those collected from the detector electronics. Real background events, stored in the database, may be mixed in with the simulated event to more closely reproduce signal data. The digitized background event is aligned in time with the simulated signal event before they are combined. The final outcome from this stage is a set of raw data objects called "digis" which are stored in the database for the reconstruction phase. These functions are performed by the SimApp application.
   
   
4. Reconstruction
   • The reconstruction stage is performed by the Bear application. For each event Bear retrieves the raw digis from the database and combines them into candidate events consisting of particle tracks, energy clusters, and probable particle identifications, among other things.

Moose

Fast Simulation